The invention relates to a polycarbonate composition having reduced subsequent yellowing of the mouldings produced therefrom on heat ageing and having good optical properties of the polycarbonate composition on processing.
The invention relates in particular to compositions comprising polycarbonate with an aromatic phosphine and a sterically hindered phenol, the polycarbonate being prepared from bisphenols and organic carbonates by the transesterification process in the melt, referred to below as “melt polycarbonate”.
In addition to its good mechanical properties, polycarbonate is distinguished, inter alia, by high transparency and brilliance of colour. One possibility for assessing the brilliance of colour is the so-called yellowness index (YI), which characterizes the degree of yellowing of the material. For high-quality polycarbonate, a low YI value is an important quality feature. Uses of polycarbonate based on bisphenol A (BPA) extend over a wide temperature range from −100° C. to about +135° C. Particularly in the case of heat ageing, i.e. on storage of mouldings at elevated temperatures of use of >100° C. over relatively long periods of weeks or months in air, polycarbonate shows so-called subsequent yellowing with increasing temperature, i.e. an increase in the YI value with time. Below, subsequent yellowing (ΔYI) designates the difference between the degree of yellowing of a moulding after heat ageing, measured as the YI value on the standard body according to ASTM D-1925, and the degree of yellowing (as YI value) of a freshly injection-moulded moulding.
A low YI value is of considerable importance both during production and in subsequent use. For colour-critical applications of polycarbonate, it is therefore desirable to produce polycarbonate mouldings having low initial YI values and to achieve as little subsequent yellowing as possible during the subsequent use under conditions of heat ageing. Subsequent yellowing occurs, for example, in the case of headlamp diffuser screens made of polycarbonate, which are exposed to continuous temperature loads of above 100° C., depending on size and design. Such applications require a material whose optical properties are substantially unchanged at a high level and which decline as little as possible over the operating time.
Polycarbonate can be prepared by various processes. The polycarbonate prepared by the interfacial process from bisphenols and phosgene in solution (IPC) first acquired industrial importance. In the transesterification process which is becoming increasingly important today, bisphenols are reacted with organic carbonates in the melt to give so-called melt polycarbonate (MPC).
This melt polycarbonate (MPC) has many differences compared with the polycarbonate prepared in interfacial process (IPC). One difference of the MPC is the more pronounced subsequent yellowing, in particular on heat ageing; which is due to higher values of the phenolic OH terminal groups. The branching structures which are present in the MPC and lead to poorer initial colours and, on heat ageing, to greater subsequent yellowing constitute another difference.
A basic possibility for minimizing the subsequent yellowing of polycarbonate is the use of heat stabilizers. The technical state of the art for heat stabilization of polycarbonate comprises substantially the use of suitable organic phosphorus compounds aromatic phosphines, aromatic phosphites, and organic antioxidants, in particular sterically hindered phenols. Frequently, the use of phosphites in combination with sterically hindered phenols is described. However, the prior art provides no information about the long-term stabilization, especially of melt polycarbonate mouldings, to heat ageing in atmospheric oxygen on use at temperatures greater than 100° C. over several weeks and months.
German Offenlegungsschrift [German Offenlegungsschrift] DE 44 19 897 A1 is concerned with the stabilization of polycarbonate, prepared by the interfacial process, to discoloration at high temperatures at 300° C. In this document, a mixture of a phosphine and a hindered phenol is mentioned as a suitable stabilizer combination for polycarbonate compositions based on polycarbonate prepared by the interfacial process. The estimate of the colour change of the mouldings is made only after the process step of the moulding materials to the mouldings, i.e. in the freshly injection-moulded state. However, the disclosure document makes no mention of so-called heat ageing, where the extruded polycarbonate compositions and the mouldings produced therefrom are exposed over a relatively long time to elevated temperature and the presence of atmospheric oxygen. The extent of any subsequent yellowing in the context of the present invention cannot be derived therefrom. In fact, there is no correlation between the thermal storage in air and the yellowness index YI zero value of the freshly injection-moulded mouldings.
It is very clear from the disclosure that the polycarbonate is one prepared by the interfacial process from phosgene and bisphenols. Such so-called solution polycarbonates (IPC) have different behaviour compared with so-called melt polycarbonates (MPC), as already indicated above, since the polycarbonates prepared by the MPC process have structures which are not present in an IPC. IPC therefore has less tendency towards colour changes under thermal load, in particular on relatively long heat ageing, than melt polycarbonates. It is therefore to be expected that an MPC will behave differently from an IPC on heat ageing.
The European Patent Application EP 559 953 A2 mentions a mixture of organic phosphorus compounds with sterically hindered phenols as a heat stabilizer mixture in the synthesis of such melt polycarbonates. In particular, phosphorus-oxygen compounds, such as, for example, phosphites, are mentioned as suitable phosphorus compounds. Pure phosphorus-carbon compounds, such as, for example, phosphines (phosphanes), in particular triphenylphosphine, are not disclosed as being suitable.
There is therefore a need for suitable melt polycarbonate moulding materials which both have little natural colouration (low YI value) after processing to give mouldings and show no significant subsequent yellowing (delta YI value, Δ YI) on heat ageing, as described above.
Surprisingly, however, it was found that in particular combinations of phosphines with sterically hindered phenols have a substantially better heat-stabilizing effect in melt polycarbonate moulding materials than combinations of phosphites with sterically hindered phenols. These MPC moulding materials according to the invention which contain a combination preferably of a phosphine with a sterically hindered phenol show reduced subsequent yellowing, in particular on heat ageing.